In order to measure the number or concentration of particulates (particles) contained in gas, an optical particle measuring apparatus (a particle counter) is often used. The optical particle measuring apparatus measures the number or concentration of particulates by sucking a predetermined amount of atmosphere from a measurement environment and detecting scattered light generated when the sucked atmosphere is irradiated with laser light.
However, if a large amount of gas is required to accurately detect particulates, the state of an environment as a measurement target may be changed. For example, in a case where a particulate concentration in a chamber of a semiconductor manufacturing apparatus is to be measured, a process environment (an atmospheric pressure, for example) in the chamber may be changed if the amount of gas sucked from the chamber is too large. In such a case, the semiconductor manufacturing apparatus cannot manufacture a semiconductor device as designed.
Furthermore, when a high concentration of particulates is contained in a measurement atmosphere, there is a risk that the optical particle measuring apparatus counts a plurality of particulates as one particulate. In such a case, the optical particle measuring apparatus cannot measure the number or concentration of particulates accurately.